1. Field of the Invention
The invention relates to a hydraulic control system for an automatic transmission. More specifically, it relates to a hydraulic control system for an automatic transmission which corrects shift controls, including a servo starting control, by learning.
2. Description of the Related Art
Japanese Patent Publication No. 179676/1993 describes a conventional control with learning for starting time of servo control of a clutch which is engaged in a shift. In servo starting control clearance between the friction pads of the clutch is decreased and the clutch is taken to the state just before start of engagement. In this control procedure, turbine rotational speed is detected as an input shaft rotation speed, and the time between a shift control start and start of speed change in input shaft rotation, which is the start of the torque phase, is measured. Then this measured time and a stored basic servo filling time are compared, and the hydraulic pressure applied to the hydraulic servo is corrected so that the measured time comes to equal the basic filling time. That is to say, when the time between shift control start and the start of change in rotational speed of the input shaft is longer than a predetermined basic time, it is determined that the filling of the hydraulic servo for the engagement side clutch (clutch which is engaged in the shift) is not sufficient, and the hydraulic pressure applied to the hydraulic servo for the engagement side clutch is corrected so that the time for starting servo control is increased. Conversely, when the time between the shift control start and the start of change in rotational speed of the input shaft is shorter than the predetermined basic time, it is determined that the filling state of the hydraulic servo is too great, and the hydraulic pressure applied to the hydraulic servo for the engagement side clutch is corrected so that the time for starting the servo control is decreased.
But the phenomenon, in which the time from start of the shift control to the rotational change in speed of the input shaft is longer than the predetermined value, is not limited to the case in which the filling of the hydraulic servo is insufficient. When the rate of increase of the hydraulic pressure for engagement after start of the servo control is too low, the time from the start of shift control to the start of rotational speed change is increased even if the starting of servo control is performed properly. According to the prior art, in this case, it is determined that the filling state of the hydraulic servo is insufficient and the time for starting the servo control is corrected to be longer. Therefore, the hydraulic servo is filled and engagement of the engagement side clutch is started too early. Then, a shift shock occurs because of a tie-up with a disengagement side clutch which is disengaged in the shift.